Plant for the combustion of impure solid fuel

ABSTRACT

In a fluidized bed plant for the combustion of impure solid fuel in the presence of an absorbent material which absorbs and neutralizes the impurities, in particular sulfur, the supply of absorbent material to the fluidized bed is controlled in dependence on the impurity content in the exhaust gases leaving the fluidized bed. To achieve a rapid control, at least part of the absorbent material is supplied to the fluidized bed in a finely ground state. The bed material may consist of coarse-grained absorbent material or a chemically inactive material, or a mixture of these. Coarse-grained absorbent material which is discharged from the fluidized bed can have its particle size reduced and be passed back to the fluidized bed.

TECHNICAL FIELD

This invention relates to a plant with a combustion chamber for thecombustion of impure solid fuel, in particular coal, in a fluidized bed.The combustion chamber may operate at atmospheric pressure or atsuperatmospheric pressure, for example at a pressure of from about 10 to20 bar. Such a combustion chamber may form part of a combined powerplant comprising a gas turbine, which is supplied with gas from thecombustion chamber, and a steam turbine supplied with steam from steamgenerators arranged in the combustion chamber and downstream of the gasturbine for recovering heat from the gases leaving the gas turbine.

BACKGROUND ART

During combustion of solid fuel, in particular coal, certain impuritiesin the fuel, especially sulfur, have to be contended with. Theseimpurities involve certain drawbacks, especially for the environment.One advantage of allowing the combustion to take place in a fluidizedbed is that the bed material may consist of, or be supplied with, anabsorbent material which absorbs and binds the impurities so that theseare deposited in the form of harmless powder or slime.

Doolmite or limestone may be used as a bed material and sulfur absorbentmaterial. Such material is supplied to the combustion chamber inparticulate form, for example with a particle size in the range of fromabout 0.05 to about 6 mm. When sulfur-containing fuel is burnt in thefluidized bed, the sulfur reacts with the oxygen of the combustion airand forms sulfur dioxide which reacts with the active component orcomponents in the sulfur absorbent material, for example calcium. Thismay occur, for example, by the absorbent material first being calcined,that is, carbon dioxide escapes from the absorbent material. After that,the surface of each particle of the sulfur absorbent material becomessulfatized, with the formation of calcium sulfate on its surface. Thesulfatization of the particles then becomes deeper and deeper from thesurface.

It has been found that very small particles of the absorbent materialare sulfatized very rapidly whereas large particles, even after a longresidence time in the fluidized bed, are still sulfatized only to aminor extent.

The supply of absorbent material takes place continuously and iscontrolled with respect to the sulfur content in the fuel. Fineparticles of the absorbent material are sulfatized rapidly and some ofthese leave the combustion chamber with the exhaust gases and arecollected in a cleaner. Coarse particles are sulfatized slowly andremain in the bed. For the bed height to be maintained constant, bedmaterial must be removed from the fluidized bed either constantly orintermittently. This withdrawn bed material is then often sulfatizedonly partially in spite of a long residence time in the bed.

The present invention aims to provide a plant for the combustion ofimpure solid fuel which makes it possible to improve the utilization of,and thus reduce the consumption of sulfur absorbent material.

DISCLOSURE OF THE INVENTION

According to the invention, a plant for the combustion of impure solidfuel comprises a combustion chamber, means for supplying said impuresolid fuel to said combustion chamber, means for supplying fine-grainedand coarse-grained absorbent material to said combustion chamber forabsorbing impurities formed in said combustion chamber by combustion ofsaid solid fuel therein, means for creating a fluidized bed of saidabsorbent material in said combustion chamber, means for withdrawingabsorbent material from said fluidized bed, and means for returning atleast some of the withdrawn absorbent material to said fluidized bedwith reduced grain size.

In use of a plant in accordance with the invention, the reduction insize of the withdrawn absorbent material exposes the inner unusedabsorbent material so that after it is returned to the fluidized bed itis again effective as an absorbent. The feedback of the absorbentmaterial of reduced grain size may suitably be controlled in dependenceon the sulfur dioxide content in the exhaust gas leaving the fluidizedbed. Because the returned absorbent material is finely-divided andtherefore efficient from the point of view of absorption, a fastregulating effect can be achieved.

In a plant in accordance with the invention having a combustion chamberwhich operates at superatomspheric pressure, the absorbent material canbe removed from the fluidized bed, and then be cooled, sieved andcrushed outside the pressure vessel which surrounds the combustionchamber. Thus, the treatment of the removed absorbent material can becarried out at atmospheric pressure.

However, by employing pneumatic feeding-out devices and return feedingdevices for the absorbent material it is possible to operate at thepressure of the plant. The reduction in grain size of the absorbentmaterial may be effected by projecting the absorbent material against aplate when it is fed back into the combustion chamber, so that thegrains are crushed. The advantages of this are that pressure reduction,temperature reduction and pressure increase can be eliminated during thereturn of the absorbent material to the fluidized bed and that it ispossible to operate substantially without movable parts in severeenvironments involving high temperature, high pressure, dust, ect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which

FIGS. 1, 2 and 4 are schematic diagrams of three different embodimentsof plants in accordance with the invention, and

FIG. 3 is a diagram of a regulating device for use in the plant of FIG.2.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the plant shown in FIG. 1, the numeral 1 designates a combustionchamber in which solid fuel is burnt in a fluidized bed 2 above whichthere is a free space 3. The combustion chamber 1 communicates via anoutlet 4 with a cleaner 5, for example of the cyclone type, in whichdust is separated from the exhaust gases from the bed 2. This dustconsists of ashes and fine fractions of absorbent material. A conduit 8supplies air under pressure to nozzles (not shown) in the bottom 7 ofthe combustion chamber 1, this air serving to fluidize the material inthe bed 2 and also serving as combustion air for the solid fuel which isinjected into the bed 2 via a conduit 11. Initiation of the combustionprocess in the chamber 1 is effected in any suitable conventional way,for example employing an oil burner (not shown). Once the combustionprocess has been initiated, it is self-supporting so long as solid fueland combustion air are supplied to the chamber 1.

The numeral 12 designates a container for fresh sulfur absorbentmaterial 13, for example limestone or dolomite. Absorbent material 13leaving the lower end of the container 12 is crushed in a mill 14 to agrain size of less than 6 mm. The crushed material leaving the mill 14is sieved in a sieve 15 and divided into fine and coarse fractions. Thefine fraction from the sieve 15 is fed to a mill 29 for further milling.The fine and coarse fractions are collected and stored in containers 16and 17, respectively. The bed 2 is supplied with fine and coarseabsorbent material from the containers 16 and 17 via batching devices 18and 19 and conduits 20 and 21, respectively, for example employingcompressed-air injection. The batching devices 18 and 19 may eachconsist of a cylindrical chamber with a rotary vaned wheel, the speed ofrotation of which can be regulated.

The batching device 19 can be controlled so as to provide a flow of thecoarse fraction of the absorbent material to the bed 2 which is eitherconstant or bears a constant relationship to the flow of solid fuel tothe bed 2. On the other hand, the batching device 18 for the finefraction of the absorbent material is controlled by a regulator 22 independence on the impurity content of the exhaust gases from the bed 2.For this purpose there is provided downstream of the bed 2, for exampleabove the bed, downstream of the cleaner 5 or in the chimney (not shown)to which the exhaust gases pass, a transducer or detector 23 whichmeasures the concentration of sulfur dioxide in the exhaust gases.Signals from the detector 23 arre led to an input circuit 24 for theregulator 22, to which also a desired value for the concentration ofsulfur dioxide in the exhaust gases is fed. Even if this desired valueis chosen as low as is practically possible, it cannot in practice beset at zero since this would probably result in a constant surplus ofabsorbent material, which is both impractical and uneconomical. Thesulfur dioxide detector 23 may be of the kind known under the Trade Mark"Thermo-Electron".

The chamber 1 is provided with an outlet 25 through which material maybe removed from the bed 2, so that the height of the bed may be adjustedduring operation of the combustion chamber. The outlet 25 leads to asieve 26 for dividing the removed material into fine and coarsefractions. The fine fraction may be considered to consist of consumedabsorbent material, which can be discharged through a conduit 50 fordisposal in a suitable manner. The grains of the coarse fraction containunconsumed absorbent material in their inner part. This unconsumed partmay amount to 50% or more. The coarse fraction is collected in acontainer 27 and is transported through a conduit 28 to the mill 29,where it is crushed into fine particles, preferably smaller than 0.1 mm.As previously mentioned, the crushed material from the mill 29 iscollected in the container 16.

In the plant shown in FIG. 2, items which are the same as, or similarto, items in the plant of FIG. 1 have been designated with the samereference numerals as in FIG. 1. In FIG. 2, the combustion chamber 1operates at superatmospheric pressure and is therefore enclosed in acontainer 6 which is capable of containing air under pressure which issupplied, via a conduit 9, to the space 10 between the container 6 andthe chamber 1. As in the case of the plant of FIG. 1, the bottom of thechamber 1 comprises nozzles (not shown) through which air from the space10 enters the bed 2 to serve as combustion air and to fluidize thematerial of the bed 2. The plant includes closed containers 30 and 31,respectively, for fine-grained absorbent material 32 and coarse-grainedabsorbent material 33. Pressure can be applied to these containers 30and 31 so that the pressure in them is as high as in the container 6.The batching of absorbent material from the containers 30 and 31 takesplace through batching devices 34 and 35, respectively, and theabsorbent material is supplied to the bed 2 through conduits 36 and 37by pneumatic conveyance. Material is withdrawn from the bed 2 through afeed-out conduit 37 and a pneumatic feed-out device 38 of the ejectortype and is transported to a return feed device 39 (also of the ejectortype) through a conduit 40. The ejector devices 38 and 39 are suppliedwith gas under pressure from the container 6 via conduits 41, 42 and 43and a pressure-increasing compressor 44. Absorbent material which isreturned to the combustion chamber 1 is projected against a plate 45 sothat the grains of absorbent material are crushed, and unconsumedabsorbent material in the returned material is exposed and becomeseffective from the point of view of absorption. Control of thewithdrawal and return of absorbent material by the ejector devices 38and 39 is carried out in dependence on the sulfur dioxide content in theexhaust gases leaving the bed 2, which is measured by the detector 23.Signals from the detector 23 are passed to the input circuit of aregulator 47 which controls a valve 48 in the conduit 42. The regulator47 can also be employed to control the batching device 34. The regulator47 may contain a pulse generator 49, as shown in FIG. 3. The pulsegenerator 49 produces operating pulses for opening and closing the valve48. The opening of the valve is determined by the pulse length. Thepulse repetition frequency may be constant, whereas the pulse length mayvary.

In the plants shown in FIGS. 1 and 2, the coarse fraction of theabsorbent material may serve both as an absorbent and as the fluidizedbed material. However, the bed may also be provided with a particulatebed material, consisting for example, of a chemically inactive mineralmaterial, for example quartz sand, or of an artificial compound, forexample, of the kind known under the trademark "MOLOCHITE". The bed isthen supplied with only fine-crushed absorbent material. This results ina certain simplification of the design. In an atmospheric fluidized bedthe design according to FIG. 1 is thus simplified to a design accordingto FIG. 4. A super-atmospheric fluidized bed will correspondingly besimplified in its design.

What is claimed is:
 1. A plant for the combustion if impure solid fuel,comprising:a combustion chamber; means for supplying impure solid fuelto said combustion chamber; means for supplying fine-grained andcoarse-grained absorbent material to said combustion chamber, saidabsorbent material being effective to absorb sulfur formed in saidcombustion chamber by combustion of said solid fuel therein; means forcreating a fluidized bed of said absorbent material in said combustionchamber; means for withdrawing absorbent material directly from ssaidfluidized bed; means for crushing said absorbent material withdrawndirectly from said fluidized bed into a fine-grained absorbent material;transducer means for producing a first signal proportional to the sulfurdioxide content in the exhaust gases leaving said fluidized bed; signalprocessing means, connected to said transducer means, for comparing saidfirst signal with a second signal proportional to a desired value of thesulfur dioxide content; and regulator means, connected to said signalprocessing means, for regulating the flow into said fluidized bed offine-grained absorbent material produced by said means for crushing, inresponse to the difference between said first and second signals.
 2. Aplant according to claim 1, further comprising a pneumatic conveyingdevice for conveying absorbent material from said means for withdrawingabsorbent material from the fluidized bed to said means for crushing. 3.A plant according to claim 2, wherein said regulator means controls saidmeans for withdrawing absorbent material.
 4. A plant according to claim3, wherein said regulator means closes and opens a valve in saidpneumatic conveying device in order to regulate the flow of absorbentmaterial from said fluidized bed.
 5. A plant according to claim 4,wherein said regulator means controls the withdrawal of absorbentmaterial from, and its return to, said fluidized bed by varying the timeduring which said valve is held open.
 6. A plant according to claim 1,comprising separate conveying devices for conveying fine-grained andcoarse-grained absorbent material to said fluidized bed.
 7. A plantaccording to claim 1, wherein the output of fine grained absorbentmaterial from said means for crushing is returned to said means forsupplying fine grained and coarse grained absorbent material.
 8. A plantaccording to claim 1, wherein said means for crushing comprises a platewithin said combustion chamber and means, connected to said means forwithdrawing, for projecting withdrawn material against said plate sothat the grains of absorbent material are crushed.
 9. A plant accordingto claim 2, wherein said means for crushing comprises a plate withinsaid combustion chamber and means, connected to said means forwithdrawing, for projecting withdrawn material against said plate sothat the grains of absorbent material are crushed.
 10. A plant accordingto claim 1, wherein said means for withdrawing comprises a firstpneumatic ejector means connected to said fluidized bed for withdrawingmaterial therefrom; and said regulator means comprises a source ofpressurized air connected to said first pneumatic ejector means and avalve between said source and said first pneumatic ejector means forcontrolling air flow to said first pneumatic ejector means.
 11. A plantaccording to claim 10, wherein said means for crushing comprises a platewithin said combustion chamber and means, connected to said firstpneumatic ejector means, for projecting withdrawn material against saidplate so that the grains of absorbent material are crushed.
 12. A plantaccording to claim 11, wherein said means for projecting comprises asecond pneumatic ejector means, connected to the output of said firstpneumatic ejector means, for projecting withdrawn material against saidplate.
 13. A plant according to claim 12, wherein said regulator meanscontrols the withdrawal of absorbent material from, and its return to,said fluidized bed by varying the time during which said valve is heldopen.